The background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
As new emerging materials, single-walled carbon nanotubes (SWCNTs) have recently attracted extensive research interest due to their specific electrical, optical and mechanical properties. For different applications, the raw SWCNT materials have to be purified and enriched, as they contain metallic (m) and semiconducting (sc) single-walled carbon nanotubes, amorphous carbon, catalyst and other impurities. For example, sc-SWCNTs can be used as the active channel materials in field effect transistors (FET) in logic circuit and other electrical devices.
Recently, conjugated polymer extraction (CPE) processes have been developed to purify single walled carbon nanotube (SWCNT) raw materials. Compared with other surfactant-based methods, such as density gradient ultra-centrifugation (DGU), gel chromatography and biphasic separation, CPE is simple, scalable and cost effective, thus possessing properties that are highly desirable for industrial applications. More importantly, the dispersed product is obtained as an organic solvent-based dispersion with relatively high tube content (e.g. up to ˜20%-50%). This leads to additional benefits in the application of SWCNT materials in device fabrication and performance.
While CPE produces enriched sc-SWCNT materials with a purity higher than 99%, the low yield for a single extraction, which renders the CPE time consuming. Furthermore, the cost of the conjugated polymer may be high due to the low yield. Another challenge is that current CPE processes have no chirality selectivity. That is, the resulting product is usually a mixture of SWCNTs with different chiralities. This is an issue in some fields of application in which narrow or even single chirality SWCNTs are required for band gap control.
Furthermore, the interaction between conjugated polymers and SWCNTs is still not clear. As such, the mechanism of the CPE process is elusive. In order to achieve enrichment, the interaction between the conjugated polymer and SWCNTs should be strong enough to form a stable complex in solvents. In addition, the solubility of the complex must be sufficient to form a stable solution. This solubility is related to the backbone and side chain structure of the conjugated polymer. Furthermore, this process should be selective such that only the sc-SWCNTs are stable in solution.
These objectives require a fine-tuned balance between the raw tube materials, the structure of the conjugated polymer and solvent. However, this balance is difficult to control. For example, to improve the yield and purity of the CPE by adjusting the polymer structure is still quite difficult. The cost to synthesize a new conjugated polymer can be quite high, especially when complicated structures are used. As such, there are no general rules on polymer structure design for a CPE process; current techniques are still based on trial and error.
While solvents can affect the purification process, the selection of solvents is limited due to solubility issues. Furthermore, while addition of a redox agent can affect enrichment process, most of the experiments and results are for aqueous system. For example, it has been found that redox chemistry and pH affect the sorting of SWCNTs in aqueous system (see, e.g., Ju, S.-Y.; Utz, M.; Papadimitrakopoulos, F., J. Am. Chem. Soc. 2009, 131, 6775-6784, “Enrichment Mechanism of Semiconducting Single-Walled Carbon Nanotubes by Surfactant Amines”; Wang, J.; Nguyen, T. D.; Cao, Q.; Wang, Y.; Tan, M. Y. C.; Chen-Park, M. B., ACS Nano 2016, 10, 3222-3232, “Selective Surface Charge Sign Reversal on Metallic Carbon Nanotubes for Facile Ultrahigh Purity Nanotube Sorting”; and Hirano, A.; Tanaka, T.; Urabe, Y. Kataura, Hiromichi, ACS Nano 2013, 7 (11), 10285-10295, “pH- and Solute-Dependent Adsorption of Single-Wall Carbon Nanotubes onto Hydrogels: Mechanistic Insights into the Metal/Semiconductor Separation”. In addition, it has been found that redox molecules trigger reorganization of a surfactant coating layer on SWCNTs in aqueous two-phase systems.
In addition, Gui H, et al., have disclosed (see “Redox Sorting of Carbon Nanotubes”, Nano Lett. 2015, 15, 1642 1646) the use of redox dopants for sorting carbon nanotubes, particularly for separating sc-SWCNT from m-SWCNT. The redox dopants are used in conjunction with polymer sorting processes, including polyfluorenes and related polymer structures. In particular, Gui et al disclose the use of vitamin E (10 mM) as a reductant and water (10% v/v) as an oxidant in polyfluorene extraction systems, and NaBH4 and HClO in polyethylene glycol/dextran systems.
In addition, Ding et al. have reported that adjusting of surface acidity of SWCNTs by addition of sodium hydroxide dramatically influences the CPE process (see J. Phys. Chem. C 2016, 120, 21946-21945). The selective mechanism of semiconducting vs metallic CNTs has been attributed to oxygen doping at ambient conditions, which preferentially cause the bundling of highly polarizable metallic (m) tubes.
US 2010/176349 discloses that redox agents may be used to separate sc-SWCNT from m-SWCNT. Such selective redox chemistry may also be used to fractionate sc-SWCNT based on chirality. The redox agents listed in this document are all metals or metal ions, especially iron, copper and gold, while there is no discussion that such redox chemistry may be used in conjunction with a polymer extraction process.
U.S. Pat. No. 7,641,883 discloses the use of benzyl viologen for the selective separation of sc-SWCNTs from m-SWCNTs, though not in conjunction with a polymer extraction process.
US 2013/336874 discloses the use of quinones for selective separation of carbon nanotubes, especially sc-SWCNTs, though not in conjunction with a polymer extraction process.
U.S. Pat. No. 8,193,430 discloses the use of a reducing agent (e.g. hydrazine) in conjunction with flavins (conjugated molecule) for selectively dispersing SWCNTs.
US 2010/11814 discloses the use of a separation medium that is involved in a redox reaction with carbon nanotubes to effect selective separation of carbon nanotubes with different characteristics. Organic redox agents include azobenzene and tetracyanoquinodimethane. There is no teaching that the method may be used in conjunction with a polymer extraction process.
US 2012/286215 and Hwang J-Y, et al. (“Polymer Structure and Solvent Effects on the Selective Dispersion of Single-Walled Carbon Nanotubes”, J. Am. Chem. Soc., 2008, 130, 3543-3553) disclose a conjugated polymer enrichment process performed in water.
For a CPE-based organic system, vitamin E (at one concentration) and water have been tested, in which no detailed yield or purity data were provided. No guidance has been provided on how to effectively adjust the concentration of the redox agent, or how select doping agents in order to fine-tune the enrichment process.
Disclosed herein is a method for improving the yield and/or selectivity of a specific kind of SWCNT, in which a dopant is added during each extraction step of a CPE process. The method can be used to separate metallic single-walled carbon nanotubes (m-SWCNTs) from sc-SWCNTs and/or to enrich sc-SWCNT having a particular chirality. For example, the use of organic dopants (instead of the oxygen/water redox couple) can be used to modulate both the yield and purity of SWCNTs from the CPE of raw tube materials.
Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures.